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Kelvin

The kelvin (K) is the SI base unit of thermodynamic temperature and the unit used in the Kelvin scale, an absolute temperature scale with 0 K at absolute zero.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

The kelvin (K) is the SI base unit of thermodynamic temperature and the unit used in the Kelvin scale, an absolute temperature scale with 0 K at absolute zero. Since 2019, it has been defined by fixing the Boltzmann constant to an exact numerical value. One kelvin is equal in magnitude to one degree Celsius, but the scales use different zero points.

Definition

The kelvin is the SI unit used to measure thermodynamic temperature. It is one of the seven SI base units and is defined through a fixed value of the Boltzmann constant.

History

The kelvin was originally tied to the triple point of water, but the SI was revised in 2019 so the unit is now defined by a fundamental constant rather than a material property.

Kelvin scale

The Kelvin scale is an absolute temperature scale that begins at absolute zero. Unlike Celsius, it does not use the degree sign, and its scale intervals are the same size as Celsius intervals.

Use in science

Kelvin is the standard unit for thermodynamic temperature in physics, chemistry, and related sciences, especially when working with absolute temperature and thermal energy.

Key facts

  • The kelvin is the SI base unit of thermodynamic temperature.
  • The Kelvin scale is absolute, with zero at absolute zero.
  • The modern definition was adopted in the 2019 SI revision.
  • The unit is named after physicist William Thomson, known as Lord Kelvin.
  • A temperature difference of 1 kelvin is equal to a difference of 1 degree Celsius.
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Canadian context

The kelvin is used in Canadian science, education, engineering, and official measurement practice as part of the SI system adopted internationally.

Frequently asked questions

What is the kelvin used for?
It is used to measure thermodynamic temperature in the SI system, especially in science and engineering.
Is kelvin the same size as a degree Celsius?
Yes. A change of 1 K is the same size as a change of 1 °C, but the zero points are different.
Why is the kelvin written as K and not °K?
The kelvin is an SI base unit symbolized by K and is written without a degree sign.
What is absolute zero in kelvin?
Absolute zero is 0 K, the lowest temperature on the Kelvin scale.
Why was the kelvin redefined in 2019?
To base the unit on a fixed fundamental constant, the Boltzmann constant, rather than on a physical reference point such as water’s triple point.

References

  1. BIPM — https://www.bipm.org/en/si-base-units/kelvin
    Supports: Current SI definition of the kelvin, symbol K, and its relation to the Boltzmann constant.
  2. NIST — https://www.nist.gov/si-redefinition/kelvin-introduction
    Supports: 2019 revised definition and explanation of the kelvin as the SI unit of thermodynamic temperature.
  3. NIST — https://www.nist.gov/si-redefinition/kelvin/kelvin-history
    Supports: Historical development of the kelvin and its earlier triple-point-of-water reference.
  4. Encyclopaedia Britannica — https://www.britannica.com/science/kelvin
    Supports: Named after Lord Kelvin, base unit of thermodynamic temperature, and relation to the Kelvin scale.